The present invention generally relates to electronic signal noise cancellation. The invention relates more specifically to a method and apparatus for canceling the effect of radiated noise on an electronically reproduced image.
A common problem in digital or electronic imaging is that during processing of an image, there is a possibility of noise contaminating the image. Typically, electronic imaging systems utilize an image sensor that converts incident light reflected from an object into electronic signals. Imperfections in the system acquiring the image may cause noise to be superimposed on the resulting electrical signal produced by the image sensor. As a result, an imperfect image is displayed.
The source of the noise could be intrinsic noise from the sensor itself or noise originating from elsewhere in the system. The noise-generating source may either radiate noise into the signal or cause noise to be conducted into the electronic signal path. Other potential sources of the noise could be power supplies, motor drivers or motors themselves. In the case of an electronic camera, the source could also be a battery charger or flash charger circuit. Generally, any power switching circuit having a transistor, diode, triac or other similar device is capable of producing sufficient noise to contaminate an actual image signal.
Since noise is annoying to users, and can produce imperfect images, and thus is undesirable in an electronic image, systems that eliminate noise are desirable. There are known methods for eliminating noise in electronic signals. U.S. Pat. No. 5,555,021 (Igarashi) discloses a compact television camera having a circuit that prevents switching noises by a transistor in a power supply circuit. Noise is prevented by switching the transmitter only during a blanking period where no video signal is being transmitted by the camera""s image pickup device. A synchronizing signal is used to insure that switching of the transmitter occurs during the blanking period. The video signal is therefore not affected by any switching noise generated by the rising or falling edge of a transistor trigger signal that prompts switching and that occurs during the blanking period.
However, the Igarashi method, and others like it, are limited in that noise can only be reduced or eliminated by waiting for blanking intervals. This is problematic because blanking intervals may not always conveniently coincide with radiated noise, meaning that the noise-generating source cannot always be effectively and efficiently controlled. Also, the noise itself may have a duration longer than the blanking interval, meaning that there is no reduction of noise that occurs outside the blanking interval window.
There are also known methods for detecting defective pixels in an image, due to noise or other causes, and thereafter replacing signals having defective pixels. U.S. Pat. No. 5,625,413 (Katoh et al.) for example, discloses video camera circuitry that corrects defective pixels of a solid state image pickup device by detecting the noise causing the defective pixels when the video camera iris is in a closed state. In the Katoh detection process, the gain of an automatic gain control circuit is increased to provide for better detection of white spot noise, i.e. thermal energy that has been converted into undesirable electronic signals or that has been injected into the electronic signal path. Execution of the detection operation occurs during the period when the camera is powering on or off to reduce user annoyance and inconvenience. The detected defective signal is thereafter replaced based on a signal of a peripheral pixel. Detection circuitry, however, increases power consumption and can cause noise itself and is therefore a problem. Also, limiting detection circuitry to operation during closed iris states and power on/off intervals means that noise occurring during other intervals is effectively disregarded.
Accordingly, there is a need in the art for a noise cancellation approach that does not require noise detection circuitry.
There is also a need for a more efficient and precise noise cancellation apparatus and method that can be activated to combat noise at any time without restrictions linked to blanking or power on/off intervals.
There is a particular need for a noise cancellation approach that is well suited to the architecture of a digital camera.
The foregoing needs and objects, and other needs and objects that will become apparent from the following description, are achieved by the present invention, which comprises, in one aspect, a noise cancellation circuit for use in canceling radiated noise generated by a noise-generating source. In one embodiment, the noise cancellation circuit is implemented in a device having an analog-digital converter that receives and converts an analog signal into a plurality of digital signals, where the noise cancellation circuit comprises a control means for controlling the timing of radiated noise, where the radiated noise is radiated into the signal path of the analog signal such that the corresponding plurality of digital signals from the analog-digital converter include a signal component derived from the radiated noise; a storage means that stores an anti-noise signal that comprises a digital representation of a complement of the signal component derived from radiated noise; and a summing means for summing the plurality of digital signals output from the analog-digital converter that include the signal component derived from the radiated noise with the anti-noise signal.
According to one feature, the control means controls the timing of the radiated noise by sending a control signal to the noise-generating source, the control signal being sent in synchronization with a clock pulse and thereby causing the noise-generating source to generate the radiated noise also in synchronization with the clock pulse.
According to another feature, each select discrete point of the anti-noise signal has an instantaneous absolute magnitude that is substantially equal to that of a corresponding discrete point of the signal component derived from the radiated noise, and a signal polarity opposite that of the corresponding discrete point of the signal component derived from the radiated noise.
According to yet another feature, summing comprises adding digital values representative of said plurality of digital signals that include the signal component derived from the radiated noise with digital values representative of the anti-noise signal some predetermined time after a given clock pulse.
According to yet another feature, the radiated noise is isolated and measured.
According to yet another feature, the control means provides information to the storage means identifying the anti-noise signal, from among a plurality of anti-noise signals in the storage means, as the anti-noise signal to be delivered to the summing means.
According to yet another feature, the control means controls the timing of the delivery of the anti-noise signal delivered to the summing means by sending a control signal in synchronization with a clock pulse.
According to yet another feature, the radiated noise is generated by a power switching circuit.
In yet another aspect of the present invention, a method for canceling the effect of radiated noise on a signal is disclosed, comprising the steps of characterizing a signal component that is derived from radiated noise; creating and storing an anti-noise signal that comprises a digital representation of a complement of the signal component derived from the radiated noise; controlling the timing of the radiated noise, the radiated noise being radiated into the signal path of a signal; providing a digital representation of the signal that includes the signal component that is derived from the radiated noise; and summing the digital representation of the signal and the anti-noise signal.